The present invention relates to surface-emitting type semiconductor lasers.
A surface-emitting type semiconductor laser is a semiconductor laser that emits a laser beam perpendicular to a semiconductor substrate, and because its in-process examination is easier than an edge-emitting type semiconductor laser, and its radiation pattern is circular, it is highly expected as a variety of sensors and light sources for optical communications. When a surface-emitting type semiconductor laser is used as a sensor or a light source for optical communications, it is desirous to obtain a laser beam having a radiation pattern with a narrow radiation angle.
As a technology for reducing the radiation angle, Japanese Laid-open Patent Application 2004-63707 discloses a method in which the size of an aperture of a current constricting section is made smaller than the size of an aperture of an emission surface for reducing the radiation angle. Also, Japanese Laid-open Patent Application 2002-208755 discloses a technique for designing an aperture diameter of a current constricting section and an aperture diameter of a laser emission area in a manner that a difference between the optical loss of a high mode and the optical loss of a lower mode becomes large.
[Patent Document 1] Japanese Laid-open Patent Application 2004-63707
[Patent Document 2] Japanese Laid-open Patent Application 2002-208755
It is an object of the present invention to provide a surface-emitting type semiconductor laser that is capable of emitting a laser beam with a narrower radiation angle.
A surface-emitting type semiconductor laser comprising:                a substrate;        a first mirror provided above the substrate;        an active layer provided above the first mirror;        a second mirror provided above the active layer;        an electrode provided above the second mirror; and        an emission surface among the second mirror, which is not covered by the electrode, wherein the electrode has a film thickness D that satisfies a formula (1) as follows.(4i+1)/8n≦D≦(4i+3)λ/8n  (1)        
(In the formula (1), i is an integer, λ is an oscillation wavelength, and n is a refractive index of a material that covers the emission surface.)
According to the surface-emitting type semiconductor laser in accordance with the present invention, the radiation angle of a laser beam can be made smaller. This effect can be obtained by setting the film thickness of the electrode provided above the second mirror in the range described above. The inventors in the present application discovered that the radiation angle of a laser beam periodically changes, and there is a correlation between the radiation angle and the film thickness of an electrode. Concretely, in examining the correlation between the radiation angle and the film thickness of an electrode, it was found that the radiation angle changed in a cycle of λ/2 with respect to the thickness of the electrode. Its details are described below. Thus, according to the present invention, because the range of the film thickness of electrodes is set based on such a consideration, a surface-emitting type semiconductor laser with a smaller radiation angle of a laser beam can be provided.
In the surface-emitting type semiconductor laser in accordance with the present invention, a specific object (hereafter referred to as “B”) provided “above” another specific object (hereafter referred to as “A”) includes B provided directly on A, and B provided above A through another object on A.
The present invention can be further implemented in the following embodiments.
The surface-emitting type semiconductor laser in accordance with the present invention may further include a dielectric film that covers at least an upper end section of the electrode on an emission surface side.
According to this embodiment, concentration of an electric field at the upper end section of the electrode on the emission surface side can be suppressed. Therefore, distortion of the mode of a laser beam emitted at the upper end section of the electrode that forms the emission surface can be suppressed. Distortion of the mode becomes a cause of an increase in the radiation angle. However, according to the present invention, the distortion of the mode can be suppressed by suppressing concentration of the electric field at the upper end section of the electrode. As a result, a surface-emitting type semiconductor laser that achieves a reduced radiation angle can be provided.
In the surface-emitting type semiconductor laser in accordance with the present invention, wherein the dielectric film may cover the upper end section of the electrode on the emission surface side, and may further be provided above at least a portion of the emission surface, and the dielectric film that is present above the emission surface may have a film thickness D′ that is defined by a formula (2) as follows.D′=(2i+1)λ/4n′  (2)
(In the formula (2), i is an integer, λ is an oscillation wavelength, and n′ is a refractive index of the dielectric.)
In accordance with this embodiment, the dielectric film is provided further to cover a portion of the emission surface. For example, when an optical fiber is connected to a laser, and after a laser beam is emitted, there may be light that is reflected on and turns from the fiber. The returning light may give noise to the emission light. However, according to the present embodiment, by setting the film thickness of the dielectric film provided above the emission surface in the range described above, such a problem can be suppressed. As a result, a surface-emitting type semiconductor laser with excellent characteristics can be provided.
In the surface-emitting type semiconductor laser in accordance with the present invention, the electrode may be formed from a plurality of divided electrodes, wherein the plurality of divided electrodes may be disposed such that a line connecting end faces thereof on the emission surface side defines a ring shape.
According to this embodiment, a part of the divided electrodes may be made to be an electrode that does not contribute to current injection, and can be made as a spare electrode, an electrode of another device that is connected to the surface-emitting type semiconductor laser or the like.
In the surface-emitting type semiconductor laser in accordance with the present invention, at least a portion of an end section of a cross section of the electrode on the emission surface side may be a curve (composed of a curve).
According to the present embodiment, because the cross-sectional shape of the upper end section of the electrode on the emission surface side is a curve, concentration of the electric field can be suppressed. As a result, effects similar to those of the above-described embodiment in which a dielectric film is provided can be obtained, and thus a surface-emitting type semiconductor laser with a reduced radiation angle can be provided.
The surface-emitting type semiconductor laser in accordance with the present invention may have a current constricting section above the active layer, wherein a diameter of an aperture defined by the current constricting section may be made smaller than a diameter of the emission surface.
In the surface-emitting type semiconductor laser in accordance with the present invention, the aperture may be positioned inside the emission surface as viewed in a plan view, wherein a difference ΔR between the diameter of the aperture and the diameter of the emission surface may be a value that satisfies a formula (3) as follows.ΔR≧d×tan (θ/2)  (3)
(In the formula (3), d is a distance from the active layer to the emission surface, and θ is an angle of broadening of light generated from the active layer.)
The surface-emitting type semiconductor laser in accordance with the present invention may further include a contact layer provided between the second mirror and the electrode.
According to this embodiment, because the contact layer is provided, current injection can be effectively conducted.